The long-term objective of this work is to more clearly define associations between clinically relevant orthodontic biomechanical manipulations, the alveolar bone turnover response and clinical outcomes as measured by tooth movement kinetics and root resorption. The focus of this application will be the impact of the timing of orthodontic appliance reactivations on these parameters. Since the timing of orthodontic appliance reactivations on humans is largely done empirically today, such data will have considerable clinical value. Five overlapping specific aims have been developed taking advantage of prior experience with a rat model for orthodontic tipping in response to 40 gram initial appliance activators. The first aim considers the events following appliance removal; the second is concerned with tandem activations allowing bone turnover to run its course prior to reactivations. In the third, fourth and fifth aims, increasing amounts of overlap between first and second activations will be modeled based on the known bone turnover dynamics. In aim three, appliances will be reactivated during the first induction period; aim four will study reactivation during the first resorption wave and aim five during the first formation wave. Multiple comparisons will be made between reactivation groups. Bone turnover will be documented with quantitative histomorphometry and chemical analyses for changes in phosphatase activities and osteocalcin concentrations in sera. Tooth movement will be assessed cephalometrically and root resorption histomorphometrically.